This application generally relates to the field of marine geophysical surveying. More specifically, the application relates to methods and equipment for marine geophysical surveying. In particular, methods and apparatus related to the operation of a source umbilical cable without functioning power cables are disclosed.
In the oil and gas exploration industry, marine geophysical surveying is commonly used in the search for subterranean formations. Marine geophysical surveying techniques yield knowledge of the subsurface structure of the Earth, which is useful for finding and extracting hydrocarbon deposits such as oil and natural gas. Seismic surveying and electromagnetic surveying are two of the well-known techniques of marine geophysical surveying.
For example, in a seismic survey conducted in a marine environment (which may include saltwater, freshwater, and/or brackish water environments), one or more seismic signal sources are typically configured to be submerged and towed by a vessel such as a survey vessel. The survey vessel is typically also configured to tow one or more (typically a plurality of) laterally-spaced streamers through the water. In a typical seismic survey, a vessel may tow a seismic signal source (e.g., an air gun or a marine vibrator) and a plurality of streamers along which a number of acoustic sensors (e.g., hydrophones and/or geophones) are located. In some instances, acoustic sensors may be secured at or near the bottom of the body of water. Acoustic waves generated by the seismic signal source may be transmitted to the Earth's crust and then, after interacting with the subsurface formation, captured at the acoustic sensors.
Likewise, electromagnetic surveys may tow equipment, including electromagnetic signal sources and streamers, in a similar fashion. For example, an electromagnetic transmitter (also referred to as an electromagnetic signal source or as an antenna) may be used to generate electromagnetic signals that are propagated into the subterranean structure, interact with subterranean elements, and then be received by electromagnetic receivers (also referred to as electromagnetic sensors) on the streamers (and/or at or near the bottom of the body of water). Data collected during a marine geophysical survey may be analyzed to locate hydrocarbon-bearing geological structures, and thus determine where deposits of oil and natural gas may be located. Some techniques of marine geophysical surveying involve the simultaneous use of seismic and electromagnetic survey equipment.
In a typical marine seismic survey, a seismic source such as a marine vibrator or an air gun is commonly used. For example, a plurality of air guns of different sizes may typically be included in an air gun array towable behind a survey vessel or another vessel. The air gun array is generally suspended by chains of selected length from a buoy, float or similar flotation device. In a typical air gun array, an individual air gun includes two electrical leads connected to a solenoid valve for firing the air gun. In addition, the air gun typically includes a high pressure air feed line.
Continuing with the example, an air gun array typically receives electrical power and air from onboard equipment of a vessel via a source umbilical cable. One end of a source umbilical cable is generally coupled to onboard equipment of the vessel whereas the other end of the source umbilical cable is connected to components and devices of the air gun array. A source umbilical cable generally includes, for example, air conduits, air hoses, and cables such as power cables (may be referred to as “power cores”) to supply air and electricity, respectively, to the air gun array from relevant onboard sources of a vessel. A source umbilical cable is generally considered a complex structure. In addition to a stress member, a source umbilical cable may include electrical, optical and hydraulic pipes or cables which allow for power, data communication, control and fluid injection between devices on the water surface (e.g., onboard devices of a vessel) and the various subsea devices.
When used with either seismic or electromagnetic surveying, a source umbilical cable commonly includes one or more electrical conductors which may be operable to provide electrical power to electrical devices and/or components of a geophysical signal source (or simply “signal source”). Such electrical conductors may be referred to as one or more power cables of a source umbilical cable, and may be a single wire, a pair of twisted wires, or multiple wires or wire pairs helically wound or otherwise bound together into a source umbilical cable. Through the power cable, an electrical power supply unit onboard of a vessel may supply electrical power to the electrical devices and/or components. An electrical power supply unit onboard of a vessel is typically coupled to a source umbilical cable to provide electrical power through the source umbilical cable to electrical devices and/or components of a source array.
In a typical geophysical survey, the more individual signal sources (either seismic or electromagnetic) are added to the source array, the larger the diameter and weight of the source umbilical cable generally is. The large diameter and weight of a source umbilical cable typically creates a great amount of frictional, turbulent and/or vibrational “drag” from the water as it is being towed behind a vessel. Not only does the drag increase fuel consumption, but the drag also induces stress on the source umbilical cable leading to rupture and leakage. The diameter and weight of a source umbilical cable are generally considered to be factors limiting the quantity of individual signal sources to be towed and the amount of offset between the vessel and the source array. Moreover, a high power requirement such as the power requirement for an acoustic ranging unit over long ranges is often another bottleneck in the operation. High power requirement generally shortens battery life for battery-powered devices and puts constraints on those devices powered inductively from the source umbilical cable.
When a source umbilical cable is damaged or ruptured, its internal components including the power cables that supply electricity to the source array may fail, resulting in interruption or suspension of the entire survey operation. Repairing faulty components of the source umbilical cable, however, is often challenging with such a large bundle of air lines, wires, and other components in one source umbilical cable. Splicing into the entire source umbilical cable is often necessary to locate and/or test electrical or pneumatic faults. Separating the faulty component from the large bundle of air lines, wires and other components, repairing the faulty component, reassembling the source umbilical cable, and repairing the rupture is a time-consuming and costly process.
Accordingly, a need exists for an improved techniques and apparatus for conducting marine geophysical survey operations. A need exists for a more robust and more reliable source umbilical cable that may tow a large quantity of signal sources. There is a continuing need for larger arrays and acquiring larger amounts of geophysical data.